Diabetes mellitus is a carbohydrate metabolism disorder caused by insufficient insulin production and or reduced sensitivity to insulin. Consequently the cells are inhibited from normal glucose utilization, resulting in abnormally high blood sugar levels and a variety of maladies. Chronic complications include diabetic retinopathy (retinal changes leading to blindness), kidney disease and frequent infection. Acute complications from diabetes may be fatal, such as “dead-in-bed syndrome” and such as “diabetic shock” wherein a diabetic person suddenly and without warning becomes temporarily blind, disoriented and or loses consciousness during normal activity. To date there is no cure for diabetes.
Although complications can often be avoided by careful management of blood glucose levels, complete control is elusive. For instance, “hypoglycemia unaware” diabetic persons who comply with medical protocols for insulin administration may nevertheless have hypoglycemic episodes and be completely unaware that diabetic shock is setting in until after the symptoms have manifested. This puts them at risk during sleep, sports, driving, and other daily activities, prevents bystanders from calling for timely medical intervention, and lack of coordination makes the hypoglycemic individual appear to be under the influence of drugs or alcohol. Consequently diabetic drivers in particular are at risk for arrest without culpability, and diabetic drivers of commercial motor vehicles often face bans abroad and onerous compliance requirements under the U.S. federal exemption program.
Blood glucose management has traditionally relied on sampling the blood; clinically the sample may be obtained by trained personnel; in non-clinical settings the diabetic individual often draws blood for a test strip by painfully pricking a finger with a lancet. The test strip is then inserted into an electronic glucose measuring device, which determines glucose levels based on electrochemistry or the degree of color change from a chemical reaction on the test strip and displays the results on the measuring device. Although the test strips are still the most reliable glucose detection method and are widely used, they are an imperfect solution. The strip method provides data only at the test times, and results may have a time lag relative to real changes in body glucose because they measure the glucose in interstitial fluid of the blood and not in the blood cells themselves. Also, because sampling is usually several hours apart the blood glucose levels can and often do change substantially for the worse without the individual being aware of it. In order to obtain continuous data, other invasive methods have been introduced employing implanted detection hardware. Common drawbacks of the invasive monitors include discomfort, complexity, potential for infection, formation of scar tissue that seals off the portal for sampling blood, and typical time lags of 20 minutes before data is reported by the device.
In recent decades over 100 major projects have been undertaken to develop reliable non-invasive blood glucose detection. However for the most part those projects have met with failure, on a spectacular scale in many cases. Those technologies and success criteria for such projects are described at length in The Pursuit of Noninvasive Glucose: “Hunting the Deceitful Turkey (John L. Smith, 2006), posted at www.mendosa.com/noninvasive_glucose.pdf. Among the techniques used have been infrared (IR) and near-infrared (NIR) spectroscopy, thermal infrared spectroscopy (TIR), Raman spectroscopy, nuclear magnetic resonance (NMR), electron spin resonance (ESR), impedance spectroscopy, dielectric spectroscopy, magneto-wave (i.e., photoacoustic) spectroscopy, and reverse iontophoresis. In the recent decade Esenaliev and Prough developed indirect but highly accurate glucose detection methods based on the use of optical coherence tomography (OCT) or ultrasound, whereby changes in tissue thickness were found to have an inverse correlation to blood glucose levels; thus they monitored tissue thickness by means of time-of flight for pulse signal echoes and found this tracked glucose levels with high fidelity relative to values obtained by test strips, the gold standard. See, for instance, U.S. Patent Publication No. 2007/0255141 by Esenaliev and Prough.
Nevertheless a variety of concerns continue to plague noninvasive technologies, as reported in Smith's book cited above. Perhaps the most important problem is that although the noninvasive methods are often excellent at correlating with known glucose levels, they are poor at predicting the exact concentrations in control experiments where the researcher is not told the absolute value of the benchmark glucose levels in a concentration plot over time. I.e., the correlation is not tracking with causation. Consequently the amount of uncertainty often exceeds the FDA's permissible limit of +20 mg/dl for accuracy in determining glucose concentration; this is quite substantial when compared to the normal range of 80 to 120 mg/dl. The results are also subject to variation caused by differences in room temperature or humidity. See, e.g., Smith at pp. 56-69.
Thus there is an ongoing and urgent need for improved non-invasive blood glucose monitors.